Ultra high performance pneumatic rubber tires typically contain rubber treads where demands for traction are very unique in that the primary traction requirement is directed to dry traction while still maintaining wet traction. This challenge often presents other issues such as having tread rubber compositions with greater internal heat generation during tire service with associated higher tire running temperature which may result in reduced tire durability. Such increase in internal heat generation is, in general, a promotion of increased hysteresis of the tread rubber composition by the inclusion of the high Tg (high glass transition temperature) elastomers. In some cases, where high Tg elastomers are used for the tread rubber composition to aid in achieving tread traction, use of such high Tg elastomers can create a relative high Tg tread rubber composition which can lead to surface cracking of the tread, including the rubber of the tread grooves.
Further, such tread rubber compositions may contain one or more traction promoting resins to aid in promoting tread traction over a range of tread temperatures.
A significant challenge for such ultra high performance tires is to provide a rubber composition to promote dry traction for the tread while maintaining wet traction through use of high Tg elastomers with attenuation of normally increasing internal heat generation of the tread rubber composition by limiting increased hysteresis of the rubber composition.
While the traction enhancement of the tread rubber composition by the traction resin content may be due to various factors, the melting point, or softening point, of the traction resin, or a combination of traction resins, is normally considered important because, as the resin melts and therefore softens, it undergoes a phase transition and its mechanical properties change.
For this invention, resins with selectively distributed softening points are to be used to better achieve this traction response over a greater operating temperature. A resin with a lower softening point (melting point) may be desirable as the tire is run from a stationary, resting position, to vehicular driving speeds where the temperature of the tread may increase from its stationary ambient temperature (e.g. 23° C.) to a higher operating temperature (e.g. 65° C.).
Therefore, the presence in the tread rubber composition of a resin with a considerably higher melting point would be desirable to promote tread traction at the higher tread temperature associated with the higher vehicular speed and a resin with a lower softening point would be desirable to promote tread traction at a lower tread temperature.
It is apparent that as the resin softens, the cured tread rubber composition containing the softened resin becomes more hysteretic as a result of the softened resin, and therefore predictively more prone to internal heat generation within the rubber composition. This means that, as the tire tread is being run at higher vehicular speeds, the tread rubber composition has a greater tendency to transform internal energy generated within the tread into heat, which results in a significant temperature increase of the rubber composition and a resultant improved traction performance for the tread at higher vehicular speeds.
Representative examples of resins which have heretofore been proposed to promote tire tread traction for tread rubber compositions are, for example, hydrocarbon-derived synthetic resins, coumarone-indene resins, rosin, rosin derivatives, terpene resins and polyester phthalate resins together with a functionalized styrene/butadiene elastomer which are referred to in U.S. Pat. No. 8,459,319.
A significant aspect of this invention is an inventive implementation of a combination of selectively distributed melting point, or softening point, resins together with a plurality of styrene/butadiene elastomers comprised of a combination of styrene/butadiene elastomer as an aqueous emulsion prepared styrene/butadiene rubber (E-SBR) and dual organic solvent solution prepared styrene/butadiene elastomers (S-SBRs).
A further significant aspect of this invention is the employment of pre-hydrophobated precipitated silica (precipitated silica pre-treated to form a pre-hydrophobated precipitated silica prior to addition to the rubber composition) to promote low stiffness (low storage modulus G′ property) at low strain (low dynamic elongation) for a tread rubber composition at low tire tread temperatures while substantially maintaining higher tread rubber stiffness (higher storage modulus G′ property) at higher tire tread temperatures.
An additional significant aspect of this invention is an employment of a significant content of small particle sized rubber reinforcing carbon black as the reinforcing filler for the tire tread rubber composition.
In one aspect, the selection of the styrene/butadiene elastomers for this invention is provided to promote a resistance to tread rubber cracking under tire load and/or tread deflection at temperatures below about 10° C. while still promoting the desired dry traction for the tread.
The plurality of styrene/butadiene elastomers are comprised of an emulsion polymerization prepared styrene/butadiene elastomer (E-SBR) having a Tg in a range of from about −30° C. to about −50° C. and a bound styrene content in a range of from about 35 to about 45 percent, a solution (organic) polymerization prepared styrene/butadiene elastomer (S-SBR-A) having a Tg in a range of from about −30° C. to about −50° C. and a bound styrene content in a range of from about 35 to about 45 percent and a solution (organic) polymerization prepared styrene/butadiene elastomer (S-SBR-B) having a Tg in a range of from about −3° C. to about −23° C. and a bound styrene in a range of from about 35 to about 45 percent, wherein the Tg's of the elastomers are desirably spaced apart from each other by at least about 4° C., desirably at least about 5° C., where the Tg of the S-SBR-A is spaced apart from the Tg of the S-SBR-B by at least 10° C. and desirably at least 20° C.
In one embodiment, said S-SBR-A has a vinyl content in a range of from about 10 to about 20 percent, said S-SBR-B has a vinyl content of in a range of from about 35 to about 45 percent and said E-SBR has a vinyl content in a range of from about 10 to about 20 percent, based on the butadiene portion of the SBRs.
A significant aspect of the inclusion of the S-SBR-A elastomer is to beneficially promote a lower composite Tg of the styrene/butadiene elastomers, since it has a lower Tg than the S-SBR-B elastomer and where the Tg of S-SBR-A is lower than the combination of the S-SBR-B and E-SBR, and to promote a lower hysteresis of the tread rubber composition to thereby promote a reduced internal heat generation of the tire tread during tire service.
Further, it is considered herein that a significant contribution of the high styrene contents of all of the selected elastomers of at least about 35 percent is envisioned to promote dry traction of the tire tread over ground contact and a broad tire tread operating temperature range.
The carbon black and silica filler reinforcement for the rubber composition is also carefully selected and is comprised of a precipitated silica pre-hydrophobated with at least one of bis(3-triethoxysilylpropyl) polysulfide and alkoxyorganomercaptosilane, desirably an alkoxyorganomercaptosilane and a carbon black of small particle size and high surface area. Such bis(3-triethoxysilylpropyl) polysulfide used to treat the silica may have an average of from about 2 to about 4 connecting sulfur atoms in its polysulfidic bridge.
A further significant aspect of the invention is an employment of at least three, desirably at least four, and optionally, even five, of said resins with the aforesaid combination of S-SBR-A, S-SBR-B and E-SBR styrene/butadiene elastomers together with reinforcing filler comprised of a significant rubber reinforcing carbon black content and said pre-hydrophobated precipitated silica.
A significant contribution of the inclusion of the pre-hydrophobated precipitated silica is envisioned to promote the aforesaid beneficial low stiffness at low dynamic elongation at ambient temperature for the tire tread rubber composition to thereby promote ultimate dry tire acceleration and engaging tire performance at ambient atmospheric temperatures. Also, the inclusion of the pre-hydrophobated precipitated silica is envisioned to promote engaging handling performance of the tire tread during relatively low ambient atmospheric temperatures in a range of from about 5° C. to about 10° C. and damp road conditions.
A significant aspect of the invention is the utilization of such resins selected from resins comprised of polyester phthalate resin having a softening point in a range of from about 20° C. to about 26° C., which is liquid or semi-liquid at about 23° C., styrene/alphamethyl styrene resin having a softening point in a range of from about 80° C. to about 90° C., gum rosin having a softening point in a range of from about 70° C. to about 100° C., desirably about 80° C. to about 90° C., coumarone indene resin having a softening point in a range of from about 90° C. to about 120° C., and alternately also including alkylphenol acetylene resin having a softening point in a range of from about 130° C. to about 150° C.
It is considered that such a unique combination of styrene/butadiene elastomers (S-SBR-A, S-SBR-B and E-SBR) together with the aforesaid combination of resins and pre-hydrophobated silica promotes a tire tread as a departure from past practice with enhanced traction and handling capability over a wide tire tread operating temperature range.
A significant contribution of the combination of resins with spaced apart softening points is considered herein to promote a varied hysteretic property of the rubber composition over a broad temperature range to thereby promote internal dynamic heat generation within the rubber composition to consequently promote traction of the tread on the ground over a wide tire tread operating temperature range.
Historically, it is appreciated that tire treads have heretofore been proposed with various elastomers having selectively distributed Tg's for various purposes such as example, U.S. Pat. Nos. 6,465,560 and 5,723,530. However, this invention is considered herein to be a significant departure from such practice particularly through the use of a combination of the aforesaid E-SBR, S-SBR-A and S-SBR-B elastomers with spaced apart Tg's, together with unique resin and filler reinforcement choices.
In the description of this invention, the terms “rubber compound”, “sulfur-cured rubber compound” or “rubber composition”, “rubber blend” and “compounded rubber” may be interchangeably used to refer to rubber which has been mixed with rubber compounding ingredients. Such terms are well known to those having skill in such art. The term “phr” is used to refer to parts by weight per 100 parts by weight rubber, as is a conventional practice.
A reference to glass transition temperature of an elastomer, or Tg, as referred to herein, as well as a reference to a resin's melting point, represents an inflection point glass transition temperature of the respective elastomer determined by a differential scanning calorimeter (DSC) at a temperature rate of 10° C. per minute by convention procedure well known to those having skill in such art.
A reference to a resin's softening point as referred to herein relates to its softening point determinable by ASTM E28-58T, sometimes referred to as a “Ring and Ball” softening point.